Slow-wave structure



Nov. 27, 1962 Filed Dec. 15, 1958 J. E. NEVXNS, JR

SLOW-WAVE STRUCTURE 4 Sheets-Sheet 1 imam.

Nov. 27, 1962 J. E. NEVINS, JR 3,066,237

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SLOW-WAVE STRUCTURE 4 Sheets-Sheet 3 Filed Dec. 15, 1958 w w x95 v OwEuuwE MJOa Nov. 27, 1962 J. E. NEVINS, JR 3,066,237

SLOW-WAVE STRUCTURE Filed Dec. 15, 1958 4 Sheets-Sheet 4 ELECTRON GUNllilimlmhlm United States Patent 3,066,237 SLOW-WAVE STRUCTURE John E.Nevins, (In, Los Angeles, Calif, assignor to Hughes Aircraft Company,Culver City, Calif., a corporation of Delaware Filed Dec. 15, 1958, Ser.No. 781,420 Claims. (Cl. 315-) This invention relates to traveling-wavetubes, and particularly to improved forms of slow-wave structures fortraveling-wave tubes.

It is well known that in the operation of traveling-Wave tubes theinteraction between the electron stream and the traveling wave causes adiminution in the axial velocity of the electron stream. The consequenceof this fact is that the relative axial velocity of the traveling waveand the electron stream become appreciably different toward thecollector end of the traveling-wave device. Accordingly, there is areduction in the effective interaction between the stream and thetraveling Wave, and a consequent reduction in the efliciency of thedevice.

A number of different techniques have been suggested for modifying theslow-wave structure so as to achieve improved interaction along thelength of the travelingwave tube. Thus, with the helix type of slow-wavestructure there has been provided a gradual decrease in the pitch of thehelix along the path of the electron stream to cause a decrease in theaxial velocity of the traveling wave to correspond to the similardecrease in the axial velocity of the electron stream. It is, however,difficult to fabricate helices of uniform pitch with the desiredaccuracy and rigidity, and even more difiicult to fabricate the desiredvarying pitch helix. It is also exceedingly troublesome and complex todesign a tapered pitch slowwave structure which has proper correctionand compensation along the structure for the parameters of thestructure, for example, its impedance, which are unavoidably altered inthe tapering scheme.

The advent of the folded waveguide and various modified folded waveguideslow-wave structures has resulted in the provision of a number ofmodifications intended to achieve the desired maximum interaction of thetraveling wave and the electron stream. The modifications employed haveusually been analogous to the change in the pitch of the helix. Thesemodifications have in most instances involved considerable modificationof the slowwave structure. In many instances, the predetermined changein the axial velocity of the traveling wave causes a number of problemssuch that although tapering of the tortuous path to be traversed by atraveling wave may provide increased interaction, it may also tend todecrease the stability and bandwidth of the traveling-wave device.

It is therefore an object of this invention to provide an improvedtraveling-wave device which has a maximum of interaction between thetraveling Wave and the electron stream passing therealong.

It is another object of this invention to provide an improved slow-wavestructure which has desirable varia tions in the phase relationshipbetween the traveling wave and associated electron stream, but which iseasy to fabricate and install.

A further object of this invention is to provide an improved slow-wavearrangement for the selective control of axial slow-wave velocity andwhich also can provide regularly and equally spaced interaction cells orcavities.

These and other objects of this invention are achieved, in oneexemplification, by a slow-wave structure which employs a plurality ofconductive discs which set off individual interaction elements definingcells or cavities or interaction cavity resonators spaced along andnormal to the axis of an electron stream. The inner periphery of thediscs, adjacent the stream, is terminated by indiice vidual ferruleswhich are supported by the discs and which are concentric with the beamaxis. The remainder of the slow-wave structure is completed byconductive spacer rings between separate adjacent pairs of discs, by ahighly conductive surface on the interior faces of the ferrules, discsand rings, and by coupling holes in the webbed portions of the discsbetween the ferrules and the radially separated spacer rings. Avariation in the periodicity of the traveling wave with respect to theelectron stream is achieved within this structure without affecting therelative positions of the successive interaction cells. In thisarrangement, the axial spacings between successive ferrules is keptsubstantially unchanged. The position of the ferrule with respect to itsassociated disc, however, is successively shifted along the length ofthe tube, because the periodicity of the ferrules is different from thatof the cells; that is, the length of the ferrule plus a gap is differentfrom the length of a cell. Also, the ferrule length may vary slightly asa function of distance along the tube. Thus, as the traveling wave ispropagated down this slow-wave structure, the point within eachinteraction cell where the interaction between the electron stream andthe traveling wave occurs is successively shifted. The distance thetraveling wave moves within the given inter action cell remainsconstant, so that the actual periodicity remains the same. The action ofthe slow-wave circuit is greatly benefited because, as seen by thetraveling-wave energy, it is purely periodic even though it appears tobe tapered to the decelerating electron stream.

The conductive discs and ferrules may be nonmagnetic if thetraveling-wave tube is focused by an external magnet. On the other hand,they may be of a magnetic material and actually be the individualmagnets or magnet pole pieces if the tube is periodically focused withmagnets which are integral with the slow-wave structure.

The novel features of this invention, as well as the invention itself,may be better understood when considered in the light of the followingdescription taken in conjunction with the accompanying drawings in whichlike refer ence numerals refer to like parts and in which:

FIG. 1 is an overall view, partly in longitudinal section and partlybroken away, of a traveling-wave tube which may be constructed with atapered slow-wave structure in accordance with the present invention;

FIG. 2 is a detailed longitudinal sectional view of a portion of thetube illustrated in FIG. 1;

FIG. 3 is an exploded view of a set of typical elements included in thestructure of an embodiment of the present mvention;

FIG. 4 is a simplified schematic type view showing a tapered slow-wavestructure constructed in accordance with the present invention; and

FIG. 5 is a longitudinal sectional view of a practical embodiment of aconventionally focused high power traveling-wave tube utilizing atapered slow-Wave structure constructed in accordance with the presentinvention.

Referring to the drawings and their description, 3. number of featuresare shown for completeness of description of an operable traveling-wavetube according to the present invention, which features are not claimedin the present application but are claimed and described more fully inapplications assigned to the assignee of the present application, forexample: Periodically Focused Traveling-Wave Tube, by D. J. Bates, H. R.Johnson, and O. T. Purl, Serial No. 764,884, filed October 2, 1958, nowPatent No. 2,985,792; Severed Periodically Focused Traveling-Wave Tube,by D. J. Bates and O. T. Purl, Serial No. 764,883, filed October 2,1958, now Patent No. 2,985,791; Periodically Focused Traveling- WaveTube With Tapered Phase Velocity, by D. J. Bates, Serial No. 764,885,filed October 2, 1958, now Patent f 3,066,237 j v 9 o No. 2,956,200; andSelf-Aligning Traveling-Wave Tube and Method, by Eugene J. Flannery andTed Leonard, Serial No. 764,886, filed October 2, 1958, now Patent No.2,957,102.

Referring with more particularity to FIG. 1, there is shown atraveling-wave tube 12 utilizing a plurality of annular disc-shapedfocusing magnets 14. In the example of this figure, these are permanentmagnets and are diametrically split, as discussed later in connectionwith the description of FIG. 3, to permit their being easily slippedbetween assembled adjacent ones of a series of ferromagnetic pole pieces16, which are shown in more detail in the later figures. The system ofpole pieces 16 and magnets 14 form both a slow-wave structure andenvelope 18.

Coupled to the right hand or input end of the slowwave structure 18 isan input waveguide transducer 20 which includes an impedance steptransformer 22. A flange 24 is provided for coupling the assembledtraveling-wave tube 12 to an external waveguide or other microwavetransmission line (not shown). The construction of the flange 24includes a microwave window (not shown) transparent to radio frequencyenergy but capable of maintaining a pressure differential formaintaining a vacuum within the traveling-wave tube 12. At the outputend of the tube 12, shown in the drawing as the left-hand end, an outputtransducer 26 is provided which is substantially similar to the inputimpedance transducer 20.

An electron gun 28 is disposed at the right-hand end, as shown in thedrawing, of the traveling-Wave tube 12 and comprises a cathode 30 whichis heated by a filament 32. The cathode 30 has a small central opening34 to aid in the axial alignment of the gun assembly with the remainderof the traveling-wave tube 12. The cathode 30 is secured about itsperiphery by a cylindrical shielding member 36 which is constructed in amanner to fold cylindrically, symmetrically back upon itself to form adouble cylindrical shield and an extended thermal path from the cathode30 to its outer supporting means. Such support and an electrical, highlyconductive path to the cathode is thus achieved while providingconsiderable thermal insulation for the cathode and filament due to theextended or tortuous path for heat conduction, as well as because of themultiple cylindrical shielding against radiant heat which is provided bythe cylinders shown. For additional details of this type of gunconstruction, see the patent to J. A. Dallons, No. 2,817,039, entitledCathode Support, issued December 17, 1957, and assigned to the assigneeof the present invention.

A focusing electrode 38 supports the cylindrical shielding member 36.The focusing electrode 38 is generally maintained at the same potentialas that of the cathode 30 and is shaped to focus the electron streamemitted by the cathode in a well-collimated, high perveance beam ofelectrons which traverses the slow-wave structure 18 andelectro-magnetically interacts with microwave energy being propagatedtherealong. The electron gun configuration is in accordance generallywith the teachings in the Patent No. 2,811,667, by G. R. Brewer, whichissued October 29, 1957, entitled Electron Gun, which is assigned to theassignee of the present invention, and to which reference may be madefor a more detailed explanation. The focusing electrode 38 is in turnsupported by a hollow cylindrical support 40 which extends from theperiphery of the focusing electrode to the right-hand end of thetraveling-wave tube 12. Its opening is hermetically sealed with ametal-to-ceramic seal 42 by means of a sealing flange 44 made of amaterial having a low coefficient of thermal expansion, such as Kovar.The right-hand extremity of the cylindrical support 40 is supported byan annular flange member 46, which also may be Kovar, and which issealed in turn to a hollow ceramic supporting tube 48. The ceramic tube48 further thermally insulates the inner intensively heated members ofthe electron gun 28 and also provides electrical insulation between thecathode-beam focusing assembly and the higher potential acceleratinganode 52. Substantially encasing the electron gun 28 and secured to thecentral or radio frequency structure of the traveling-wave tube 12 is ahollow cylinder 50, which may be Kovar, to which is sealed the ceramiccylinder 48, thus completing the vacuum envelope about the right-handend of the traveling-wave tube 12.

At the left-hand end of the tube 12, as viewed in FIG. 1, there is showna cooled collector electrode 60 which has a comically-shaped innersurface 62 for collecting the electrons from the high power electronstream and dissipating their kinetic energy over a large surface. Thecollector electrode is supported within the end of a water jacketcylinder 64 which is in turn supported by an end plate 66. A waterchamber 68 is thus formed between the outer surface of the collectorelectrode 62 and the inner cylindrical surface of water jacket 64. Awater input tube 70 supplies cool water to this chamber and a wateroutput tube 72 exhausts the heated water out of the water chamber 68.Thus, considerable power may be dissipated without destruction of thecollector electrode. Although water has been specified, obviously, otherliquids or gases may be used as coolants.

The end plate 66 is sealed to a supporting cylinder 74, which may be 0EKovar, and which is in turn sealed to a ceramic insulating cylinder 76.This ceramic insulating cylinder 76 is sealed at its opposite end toanother Kovar supporting cylinder 78, which is in turn supported andsealed to the slow-wave structure end disc 80. The collector 62, the endplate 66, the supporting cylinders 74 and 78 and the ceramic insulatingcylinder 76 are all coaxially supported in alignment with the axis ofthe traveling-wave tube 12.

For vacuum pumping or out-gassing the traveling-wave tube 12, adouble-ended pumping tube 86 is connected to both of the input andoutput waveguide transducers 20 and 26. Out-gassing during bake-out ofthe entire traveling-wave tube 12 is thus achieved as rapidlyaspossible. After the out-gassing procedure, the tube 86 is separatedfrom the vacuum pumping system by pinch-- ing off the tube at the tip88.

The traveling-wave tube of the present invention may be severed into anumber of amplifying sections 90, 92,. 94, 96 and 98. Each of theamplifying segments or sec* tions is isolated from the others by anisolator or termination section 100, 102, 104 or 106. The structure ofthese isolating sections will be discussed in detail in connec-- tionwith FIGS. 2 and 4. It sufiices at this point to dcscribe their functiongenerally as providing a substantially complete radio frequencyisolation between adjacent sections of the slow-wave structure 18 whileat the sametime allowing the electron stream to pass straight throughthe entire length of the traveling-wave tube 12. Each amplifying sectionthus provides an optimum gain while; providing freedom from oscillationsdue to regeneration. The loss in gain due to each of these isolationsections is; of the order of a few decibels and is a low price to payfor the large overall gain and power handling capabilities; of atraveling-wave tube constructed in accordance withthe present invention.It should be noted that although. the isolation sections providesubstantially complete: radio frequency isolation between adjacentamplifying, sections, the electron stream is modulated at the output ofeach amplifying section. The stream thus modulated, as it enters thesubsequent amplifying section launches a; new wave therein which isfurther amplified by the interaction between the new traveling wave andthe electron stream. Thus there is provided unidirectional cou-- plingthrough the electron stream between adjacent amplifying sections.

Referring with more particularity to FIG. 2, there is shown a detailedsectional view of a portion of the traveling-wave tube of FIG. 1. Theferromagnetic pole pieces 16 are shown to extend radially inwardly toapproximately the perimeter of the axial electron stream. Disposedcontiguously about the electron stream in each case is a short drifttube 110. The drift tube 110 is in the form of a cylinder or ferruleextending axially along the strea and supported by the pole piece 16.

Adjacent ones of the drift tubes 110 are separated by a gap 112 whichfunctions as a magnetic gap to provide a focusing lens for the electronstream and also as an electromagnetic interaction gap to provideinteraction between the electron stream and miscrowave energy traversingthe slow-wave structure.

At a radial distance outwardly from the drift tubes 110 each of the polepieces 16 has a short cylindrical extension 114 protruding from itssurface. The extension 114 provides an annular shoulder concentric aboutthe axis of the tube for aligning the assembly of the component elementsof the slow-wave structure 18. Disposed radially within the extension114 is a conductive, nonmagnetic circuit spacer 116 which has the formof an annular ring having an outer diameter substantially equal to theinner diameter of the cylindrical extension 114. The axial length of thespacer 116 determines the axial length of the microwave cavities 118which are interconnected along the length of the slow-wave structure 18.It is thus seen that the slow-wave structure may be assembled andself-aligned by stacking alternately the pole pieces 16 and the spacers116. Each spacer 116 has two annular channels 120 in which, during thestacking procedure, a sealing material, such as a brazing alloy, isplaced. When the slow-wave structure 18 is as sembled, it may be placedin an oven within a protective non-oxidizing atmosphere and heated sothat the brazing alloy in the channel 120 melts and fuses or brazes theadjacent members of the slow-wave structure 18 together to form avacuiun-tight envelope. The spacers 116 are fabricated of a nonmagneticmaterial, such as copper, thus providing a highly conductive cavity wallwhile not magnetically shorting out the focusing gaps 112. The entireinterior surfaces of the cavities are preferably plated with a highlyconductive material, such as a thin silver or gold plating 121.

For interconnecting adjacent interaction cells, a coupling hole 122 isprovided in each of the ferromagnetic pole pieces 16, the more detailedshape and orientation of which will be described in connection with thedescription of FIG. 3 below. Also disposed between adjacent pole pieces16 are the focusing magnets 14 which are annular in shape and fitangularly or azimuthally sym' metrically about the cylindrical shoulderextensions 114. The magnets 14 may be diametrically split to facilitatetheir being applied to the slow-wave structure 18 after it has beenotherwise assembled. The axial length of the magnets 14- issubstantially equal to the axial spacing between adjacent pole pieces16, and their radial extent is approximately equal to or may be, asshown, greater than that of the pole pieces 16. To provide the focusinglenses in the gaps 112, adjacent ones of the magnets 14 are stacked withopposite polarity, thus causing a reversal of the magnetic field at eachsuccessive lens along the tube.

Referring to a typical isolator section 1120, there is shown asubstantial continuity of the pole piece-magnetspacer assembly. However,thepole pieces 124 at either end of the isolator section and the spacer126 are somewhat modified with respect to pole piece 16 and spacer 116,respectively, which will be shown with greater clarity in FIG. 3. It issufficient here to point out that attenuating material, which may be inthe form of lossy ceramic buttons 12% which extend from within acoupling hole 122 through the special spacer 126 and partially into thewall of the pole piece 124 opposite the coupling hole. The spacer 126forms a pair of modified cavities 130 which lie opposite respective onesof the coupling holes 122 and which are substantially filled with thelossy attenuating material.

The two cavities 131! are substantially isolated from each other by ashort circuiting vane, shown in a later figure, and are isolated frominteraction with the electron stream by means of a central portion ofthe special spacer which has the form of a ring having substantially thesame radial dimensions as the drift tubes and which extends between twoof the drift tubes 110 as shown in a manner to substantially shield theelectron stream from the slow-wave structure in the region of theisolator section 1%.

Along the length of slow-wave structure 18 the individual microwavecavities or interaction cells 118 are coupled to the electron stream bymeans of the gaps 112 between adjacent ones of the drift tubes 110'. Inaccordance with the present invention, the position of the individualcoupling gaps 112 in each cell with respect to the axial center of theend walls of that cell may be varied in a manner to provide a taper ofthe slow-wave structure. For example, in each section of thetraveling-wave tube 12, or particularly in the output section, it may beadvantageous, as discussed above, to correct for the deceleration of theelectron stream as it gives up energy to the traveling waves traversingthe slow-wave structure. As also discussed above, this deceleration isnormally inherent and results in a loss of synchronism between themodulated electron stream and the traveling waves which in turn resultsin a decrease in efficiency of the tube. One way to provide suchtapering is to actually change the periodicity of the slow-wavestructure so that, for example, the microwave cavities 118 are closertogether so that the electron stream, though it is decelerating, willcontinue to interact with successive ones of the interaction cells witha constant periodicity. However, such changing of the geometricparameters of the interaction cells gives rise to a great many problemswhich makes the process extremely complicated since altering thegeometric parameters of the cells affects the electric parameters, suchas the impedance of the circuit. In accordance with the presentinvention, the geometric parameters of the individual interaction cells118 which affect the electromagnetic properties thereof are not altered.The drift tubes 110 may be shortened slightly and are shifted upstreamto the left progressively along the tube so that the electron stream mayexperience interaction through the gaps 112 at a substantially constantperiodicity, even though the stream is decelerated. In other words, theaxial placement of the coupling gap 112 with in each of the interactioncells 118 does not affect the electrical properties of the interactioncell, but the placement of the gaps does affect the point at which theelectron stream interacts with the particular interaction cell.

in FIG. 2 it may be seen that the drift tubes 110 in the section of theslow-wave structure 18 disposed to the left of the isolator section 101}have been shifted to the left, that is, upstream so that in the lastcell of that section, viz., that adjacent the isolator section 100, thecoupling space 112 is all the way to the left in its respectiveinteraction cell. It may also be seen that in the amplifier section tothe right of the isolator section the drift tube 110 in the firstinteraction cell is disposed all the way to the right of that cell, andthat in subsequent interaction cells it is shifted progressively to theleft as shown. Other practical embodiments and a more schematic andsimplified version of the invention is described in connection withFIGS. 4- and 5.

Referring to FIG. 3, one set of the plurality of pole pieces, magnets,spacers and drift tubes or ferrules is shown for purposes of describingmore clearly how the individual elements of the slow-wave structure 18are fabricated and assembled. A typical pole piece 16 is shown twice inthe figure, once in plan and once in side elevation. A typical magnet 14and a typical spacer 116 are shown in side elevation only.

Referring to the side elevation view of the pole piece 16, it may beobserved that the orientation thereof is concentric about the electronstream. Substantially immediately surrounding the electron stream andsupported by the pole piece 16 is the short ferrule or drift tube 110which extends axially along the electron stream. As indicated in FIGS.2, 4 and 5, the axial position of the drift tube 110 with respect to thepole piece 16 may vary from pole piece to pole piece successively alongthe slow-wave structure 18. Again, additional discussion concerning theaxial placement of the drift tubes 110 is deferred to the description ofFIGS. 4 and below.

The pole piece apart from the drift tube 110 extends radially outwardlytherefrom as shown. Positioned concentrically about the drift tube 110and radially separated therefrom are the cylindrical shoulder extensions114 which extend axially outwardly from either face of the pole piece16.

The outer diameter of the cylindrical extension 114 supports thefocusing magnet 14 coaxially about the electron stream while the innerdiameter of the extension 114 rests against the outer periphery of thespacer 116. The inner diameter of the spacer 116 determines the outerradial dimension of the interaction cell which is formed betweenadjacent ones of the pole pieces 16. Before assembly, a sealing materialis placed in the channels 120 which are continuous annular grooves inthe end surfaces of the spacers 116. As indicated previously, inconnection with the description of FIG. 1, the magnets 14 may bediametrically split into an upper half 14a and a lower half 14b tofacilitate their insertion or replacement after the tube is otherwiseassembled.

An off-center coupling hole 122 is provided through each of the polepieces 16 to provide the transfer of radio frequency energy from cell tocell along the slow-wave structure 18.

The size, shape and orientation of the coupling hole 122 may be moreclearly seen in the plan view thereof at the left hand end of FIG. 3.The drift tube 110 is shown as having an inner radius r slightly largerthan the radius of the electron stream and having an outer radius rwhich substantially defines the inner radius of the interaction cell.The kidney-shaped coupling hole 122 may be formed by an end mill havinga diameter extending from r to r The end mill is pressed through thethickness of the pole piece 16 centered upon the arc of a circle 132.The end mill, or preferably the work, may then be swung along this arekeeping its center on the circle 132. The work is rotated through an arcof angle a where a may be any angle between zero degrees and, forexample, approximately 60. Thus, the kidney-shaped coupling hole 122lies between a radius r and 1' and has circular ends of diameter r rDisposed radially outwardly from the coupling hole 122 is a cylindricalshoulder extension 114, the inner radius of which is designated r and issubstantially equal to the outer radius of the spacer 116. The innerradius r of the spacer 116 determines the outer dimension of the radiofrequency interaction cell. The outer radius of the extension 114,designated as r is substantially equal to the inner radius of the magnet14. The outer radius of the pole piece 16 is designated r and the outerradius of the magnet 14 is designated r For angular alignment purposesduring assembly, one or more sets of holes 134 are provided through thepole pieces 16 to hold them in a predetermined angular position withrespect to each other. A reference notch 136 may be provided on theperiphery of each of the pole pieces 16 in order that one may alwaysknow from an observation of the outer surface of the assembled tube whatthe angular orientation of each pole piece is. In the example describedhere, the notch is always provided opposite the center of thekidney-shaped coupling hole 122.

The advantages of the slow-wave structure formed of which is the samedirection successively alternating spacer rings 116, 117 or 126 and polepiece discs 16 joined together, as by brazing, include the fact that acombined slow-wave structure may be provided which is hermeticallysealed and extremely rugged. At the same time, this structure does notrequire special aligning rods or other aligning devices. It is veryprecisely positioned, so that focusing of the electron stream may beaccomplished with members which extend to the very edge of the electronstream, thereby increasing the efiiciency of the tube. The constructionof the device from separate ceramic or metallic shapes of inherentlystrong configuration means that problems of tube deterioration ordestruction due to heating or extreme environmental conditions areminimized. The shoulders 114 in the discs 16 are concentric with thedesired electron beam path. Therefore, when the outer periphery of therings 116, 117 and 126 registers with the shoulders 114, all the membersare accurately positioned and concentric. Furthermore, when the brazingmaterial is fused, the result is a rugged air-tight envelope.

In the operation of the traveling wave tube 12, microwave energytraverses from right to left along the slowwave structure, beingamplified first in section 98 due to its interaction with the electronstream. Near the output of this amplifying section, the traveling wavehas grown and has caused considerable density modulation in the electronstream. At the first isolator section, section 106 in the drawing, theradio frequency energy in the slow-wave structure 18 is substantiallycompletely absorbed. However, the modulated electron stream passes oninto the next amplifier section, section 96, where it launches a newtraveling wave in that section. The new traveling wave grows and isamplified by the electron stream until reaching its output end at theisolator section 104. The electron stream is further modulated and therf energy in the slow-wave structure is again completely absorbed. Thisprocedure is repeated until the highly modulated electron stream entersthe output amplifier section through the isolator section 100 andlaunches a high energy traveling wave upon the output section 90 of theslow-wave structure 18. The output of this final section is fed into theoutput waveguide through the transducer 26;

The isolator sections 100, 102, 104 and 106 each represent a loss of afew decibels of amplification. However, overall they vastly increase theamount of power amplification or gain which may be achieved in a singletraveling-wave tube. The isolation sections isolate adjacent amplifyingsections, thereby to preclude instability and oscillations due toreflections and to too great an amplification in a single traveling-wavetube section.

Having considered the nature of the construction of this novel slow-wavestructure and traveling-wave tube, the arrangement by which maximum beamand traveling wave interaction is provided may be explained withreference to FIG. 4. FIG. 4 is a simplified schematic representation ofa slow-wave structure which utilizes tapering in accordance with thepresent invention. The general form of intercoupled cavity slow-wavestructures is illustrated by a number of conductive walls or fins 142equally spaced to form interconnected cavities 143 along the path 144 ofan electron stream. The electron stream is assumed to travel from leftto right in the drawing, 146 as the axial movement The electron streammoves be tween an electron gun 148 at the left-hand end and a collectorstructure 150 at the right-hand end.

A separate ferrule or drift tube 152 lies within and may be supported byeach of the cavity walls or fins 142 and concentrically encompasses theelectron stream. Each of the cavity walls 142 is apertured at some point154 spaced radially apart from the respective drift tube 152 to provideenergy coupling to the adjacent cavity. With this arrangement, it may beseen that the geometrical dimensions of the separate cavities 143 arethe same. That of the traveling wave.

is, the characteristic length of each cavity 143 as defined by theseparation between adjacent walls is constant. The distance betweenadjacent ferrules 152 which defines the gap 156 of each cavity ismaintained constant also. The length of each of the drift tubes 152 may,however, vary slightly without electromagnetically affecting itsrespective cavity in order to facilitate the shifting of the drifttubes.

The distance which is varied in this structure, however, is a distance158 between the centers of the adjacent gaps. This may also bevisualized as a shifting of each gap with respect to the cavity. Notethat the shifting is in the direction toward the electron gun 148 sothat in effect the ferrules or drift tubes are shifted upstream withrespect to the electron stream. The maximum amount of tapering of thistype which can be done is determined by the length of the drift tubesand by the gap spacings with respect to the cavities, as well as by thedegree of taper or the amount of incremental shifting which is desiredbetween cavities.

The operation of the general intercoupled cavity structure 140illustrated in FIG. 4 provides maximum interaction between the electronstream and the traveling wave. The traveling wave is isolated, in asense, frorri the electron stream over an appreciable portion of itstravel through each of the cavities 143. Interaction occurs between thetraveling wave and the stream only at each of the gaps 156 in thecavities. The interaction results in the charged particle stream givingup some of its energy to the traveling wave because of the slightlygreater average axial velocity of the electron stream. Thiselectromagnetic coupling in turn slows down the electron beam. It isimportant to note that if the traveling wave slow-wave structure isperiodic throughout, it will have greatest stability and be easiest tocontrol. This constant periodicity is maintained, in the presentinvention, even though the interaction point of the gaps 156 is adjustedalong the length of the slow-wave structure to compensate for thedecrease in the axial velocity of the electron stream.

As the beam gives up energy, the time of passage from gap to gapincreases, if the gaps remain equally spaced. The tapering orincremental shifting here of the gaps 156 relative to the cavities 143,however, compensates for this slowing down. In effect, the electricallength of each cavity as seen by the stream is kept equal to the phaseshift per cavity.

Whereas, the arrangement of FIGS. 1 through 3 illustrates the operationof the invention with periodic permanent magnet focusing, non-periodicand electromagnet focusing may also be employed, as is illustrated inFIG. 5. As shown therein, a traveling-wave tube 160 having an electrongun end 162, a collector end 164 and an intermediate slow-wave structure166 may have an electromagnet 168 encompassing the slow-wave structure166. The slow-wave structure 166 is again of the intercoupled cavitytype, and an input 170 and output 172 for the traveling wave are coupledto the extremities of the slowwave structure 166. As described inconjunction with previous arrangements, the intercoupled cavities 174include drift tubes 176 having fixed gap spacings 178 relative to eachother and concentric with the electron stream.

The necessary focusing of the electron stream for this arrangement isprovided because the permeability of the intercoupled cavity structureis not sufiicient to provide shunting of the magnetic focusing fieldcreated by the electromagnet. The gaps 178 between the drift tubes 176,are, however, successively shifted upstream, in accordance with theprevious description, to keep the periodicity of the traveling wavestructure constant while utilizing the optimum, substantially equallytime spaced, interaction points in or along the electron stream.

Thus there has been described an improved slow-wave structure fortraveling-Wave tubes which is extremely It) easy to manufacture andwhich has important operative advantages but which does not increase thecomplexity of the slow-wave structure. Maximum interaction between thetraveling wave and an electron stream can be achieved by creating anindependence between the desired constant periodicity of the slow-wavecircuit and the tapered periodicity of the stream interaction gapswithout the introduction of phase instabilities or traveling wavedegradation.

What is claimed is:

l. A slow-wave structure for providing interaction between anelectromagnetic wave being propagated thereby and a stream of chargedparticles being projected along a predetermined path comprising: aseries of electromagnetic elements each defining an interaction cavitydisposed in sequence along said path, each of said interaction cavitiesbeing electromagnetically exposed to said stream at an axial positionwithin said interaction cavity, said axial position with respect to theaxial center of each of said interaction cavities being shiftedprogressively upstream along the length of said slow-wave structure in amanner to effectively taper the interaction along said slow-wavestructure while not otherwise affecting the physical parameters of saidinteraction cavities.

2. A slow-wave structure having a changing apparent periodicity to theelectron stream and an actually constant periodicity, said slow-wavestructure comprising: a plurality of elements each defining aninteraction cavity positioned along the axis of the electron stream,said interaction cavities being defined by a plurality of regularlyspaced planar Web members exending radially outwardly from a pointadjacent the electron stream and ring member disposed between andinterconnecting adjacent web members, said ring members being radiallyspaced apart from said electron stream; and a plurality of drift tubesegments, each encompassing the electron stream and concentrictherewith, and each coupled to a different web member, the relativeaxial position of said rift tubes with respect to said regularly spacedweb members being shifted along the length of the slow-wave structure.

3. A slow-wave structure for providing interaction between anelectromagnetic Wave being propagated therealong and a stream of chargedparticles traversing a predetermined path, said structure comprising: aseries of planar conductive disc members disposed transversely to andconcentrically about said stream, said members being substantiallyequally spaced and being the axial termini of adjacent electromagneticelements each defining an interaction cavity, and a like series ofconductive ferrules having an axial length greater than the axialthickness of said planar disc members, individual ones of Which aresupported in a predetermined axial position by and with respect torespective and individual ones of said disc members, said ferrules beingaxially spaced to provide interaction coupling between said stream andrespective ones of said interaction cavities, said conductive ferrulesalong said path being progressively shifted upstream with respect to itsrespective cavity to provide a slow-wave structure which appears to theelectron stream to be tapered whereby the time between interaction withsuccessive ones of said interaction cavities as experienced by thedecelerating stream of particles remain substantially constantthroughout the length of said structure.

4. A high power periodically focused traveling-wave tube having atapered slow-Wave structure comprising: means for producing an axialelectron stream along the length of said tube; a plurality of radiofrequency elements each defining an interaction cavity intercoupledalong the length of said tube, each comprising a ferromagnetic drifttube disposed contiguously about said electron stream; a ferromagneticpole piece forming an end wall of each cavity and a nonmagneticconductive short hollow cylindrical spacer disposed concentrically aboutsaid electron stream between adjacent ones of said pole pieces; an

annular focusing magnet having an axial length substantially equal tothat of said spacer and disposed concentrically thereabout, said polepiece extending radially from said drift tube to approximately theradial extremity of said magnet and being relieved forming a couplinghole therethrough between said drift tube and said spacer, said spacerbeing hermetically bonded along its end to said pole pieces wherebyalong the length of said slowwave structure a vacuum envelope isprovided, said drift tube having an axial length substantially greaterthan the axial thickness of said pole piece, the drift tubes associatedwith said cavities toward the output end of said tube being axiallyshifted with respect to said pole piece toward the input end of saidtube whereby said electron stream traverses a progressively shorterdistance between interaction cavities, thus effectively tapering thetraveling-wave tube in a manner whereby said electron stream, whilegiving up a portion of its energy thereby slowing down, continues todeliver energy to the radio frequency traveling waves traversing saidslow-wave structure.

5. A slow-wave structure for providing electromagnetic energy beingpropagated by said slow-wave structure and a stream of charged particlestraveling in a given direction along a given path, said slow-wavestructure comprising: a series of conductive, substantially planarmembers disposed sequentially along said path perpendicularly thereto,and a series of conductive ferrules supported in axial registrycontiguously about said path by respective ones of said conductiveplanar members, adjacent ones of said planar conductive membersdetermining axially a series of interaction cavities, adjacent ones ofsaid conductive ferrules being spaced from each other to provideinteraction coupling between said stream and said interaction cavities,said conductive ferrules being shifted upstream with respect to theirrespective supporting planar conductive members in a manner such thatthe coupling spacing between successive adjacent ones of said ferrulesis progressively shifted upstream, thereby effectively to taper saidslow-wave structure while not otherwise affecting the geometricparameters of said interaction cavities.

6. A slow-wave structure for propagating an electromagnetic wave inenergy exchange relation with a stream of charged particles projectedalong an axial path comprising: a series of electromagnetic elementseach defining an interaction cavity disposed in sequence along saidpath, each of said cavities being axially terminated by a conductiveWall disposed transversely to said path at opposite axial ends of saidcavity, and each of said cavities being radially determined on its outerextremity by a conductive wall extending between said axial end wallssubstantially orthogonal thereto and being terminated on its innerextremity by a surface comprising a conductive ferrule supported by oneof said end walls and extending into said cavity in a manner whereby aseries of ferrules corresponding one to one with said conductive wallsare disposed contiguously about said path with a gap between adjacentones of said ferrules to provide electromagnetic coupling between saidstream of charged particles and said cavities, the relative axialposition of successive ones of said ferrules with respect to itsrespective supporting conductive wall being shifted toward the upstreamdirection of said stream of charged particles to thereby provide atapering for said slow-wave structure.

7. A slow-wave structure for traveling-wave tubes of the characterhaving a longitudinal axis along which an electron stream is projected,said slow-wave structure comprising: a plurality of planar ferromagneticdiscs, said discs being positioned at successive individual points alongthe electron stream axis of said traveling-wave tube, and substantiallynormal thereto and concentric therewith, each of said discs having acentral apertured portion about the electron stream; ferromagneticferrule members, each of said ferrules defining the inner aperture of adifferent ferromagnetic disc and each being concentric with the electronstream, said ferrules being of like length and having like axialspacing, the axial relationship along the electron stream of successiveferrules with respect to the associated magnetic disc being successivelyshifted along the length of the traveling-wave tube; a plurality ofconductive spacer elements, each positioned between a different adjacentpair of ferromagnetic discs and spaced radially apart from the ferrulesthereof; and highly conductive surfacing disposed in the interiorportions of said spacer rings and on said ferrules and the interiorportions of said ferromagnetic discs.

8. A traveling-wave tube slow-wave structure comprising: a plurality ofradio frequency interaction cavities disposed successively along thepath of the electron stream of the traveling-wave tube, each of saidinteraction cavities having the general form of a pair of axiallygapped, separated drift tubes and contiguously encompassing the electronstream; a pair of conductive supporting discs extending radiallyoutwardly from each of the drift tubes and a spacer ring between theadjacent discs and disposed substantially concentric with the drifttubes, said interaction cavities being arranged to have differentapparent, as seen ,by the electron stream, and actual, as seen by thetraveling wave, periodicities in a selected pattern, at least some ofsaid cavities having a successive variation in the axial position of thedrift tubes therein with respect to the supporting discs, the gapbetween the drift tubes remaining the same while the position of thespacing axially with respect to the discs is successively shifted withinthe cavity from a point adjacent one disc to a point adjacent therelatively opposite disc.

9. A traveling-wave tube comprising: an electron gun positioned at oneend thereof; means for projecting an electron stream of predeterminedaverage diameter along the longitudinal axis of said traveling-wavetube; input means including an input microwave transmission means forimpressing upon the input end of said tube a microwave signal to beamplified; output means including an output microwave transmission meansfor extracting an amplified signal from said traveling-wave tube; aplurality of electrically conductive magnetic disc members axiallypositioned along the electron stream path and being spaced apart by asubstantially constant axial distance, each of said discs comprising aninner axially extended drift tube portion defining a central aperturefor passage of the electron stream, adjacent ones of said drift tubesbeing axially separated at a predetermined point between adjacent onesof said discs to permit interaction between said microwave signals andsaid electron stream, said predetermined point being progressivelyvaried with respect to its axial position between said discs along saidtraveling-wave tube to provide a tapered slow-Wave structure whereby asthe electron stream is decelerated it continues to interact to a maximumdegree with the microwave energy traversing the length of saidtraveling-wave tube, each of said disc members also including an innerweb portion extending radially outwardly from said drift tube and havingan axial intercoupling' aperture therethrough; a plurality of conductivenonmagnetic annular cylindrical spacers, each concentric with saidlongitudinal axis and hermetically sealed between adjacent ones of saiddisc members at a radial position such that the inner surfaces of saidspacer define the outer cylindrical surfaces of the radio frequencyinteraction cells between adjacent ones of said disc members; highlyelectrically conductive surfacing disposed over the outer surfaces ofsaid drift tubes and over the inner web portion of said disc members forproviding a continuing surface of high electrical conductivity along thesurfaces defining the slow-wave structure of said traveling-wave tube; aplurality of split annular permanent magnets concentric with thelongitudinal axis of said tube and at least coextensive with the outerportions of said disc members radially beyond the outer surfaces of saidspacers, said split annular magnets having an axial length aong saidtube substantially equal to that of the spacer between respective onesof said disc members and substantially registering with the outerextremity, each of split annular magnets being positioned between adifferent successive pair of said disc members and being magnetizedsubstantially parallel to said longitudinal axis to form magnetic polesof opposite polarity on the axial extremities of adjacent respectiveones of said drift tubes, thus providing periodic focusing lenses forthe electron stream passing con-tiguously therewithin.

10. A periodically focused traveling-wave tube ineluding a plurality ofannular focusing magnets and disc-like ferromagnetic pole piecesarranged alternately in sequence along the length of said tube, eachsaid pole piece extending radially outwardly to approximately the radialextremities of said magnets and extending inwardly to a point radiallycontiguous to the electron stream of said tube and having a shortaxially extending cylindrical extension protruding from the plane ofsaid disc-like pole piece at the inner region thereof about saidelectron stream to form a drift tube therefor, said traveling-wave tubealso including nonmagnetic conductive annular spacer ring members havinginner and outer diameters between that of said drift tube and the outerdiameters of said pole piece individually disposed between a pair ofadjacent pole pieces for maintaining critical axial spacingtherebetween, each said spacer ring member having an inner diameterequal to a predetermined diameter of a desired radio frequencyinteraction cavity, the pole pieces having coupling holes therethroughbetween said drift tube and said second cylindrical extension, wherebythere is formed an interaction cavity determined by a pair of said polepieces separated by said spacer, the inner surface thereof defining theouter cylindrical surface of said cavity, and said drift tubes extendingaxially from each of said pole pieces defining the inner cylindricalsurface of said cavity, said drift tubes being separated by a gapdisposed at a predetermined position Within said cavities for couplingradio frequency energy in said interaction cavities with said electronstream, as well as a magnetic focusing lens disposed contiguously aboutsaid electron stream, said predetermined position of said gaps beingshifted progressively upstream along the length of said travelingwavetube toward its output end to provide an effectively tapered slow-Wavestructure while maintaining the actual periodicity of said interactioncells as well as their electrical parameters substantially constantalong the length of said traveling-Wave tube.

References Cited in the file of this patent UNITED STATES PATENTS2,636,948 Pierce Apr. 28, 1953 2,841,738 Pierce July 1, 1958 2,847,607Pierce Aug. 12, 1958 2,922,920 Convert Jan. 26, 1960 2,956,200 BatesOct. 11, 1960 FOREIGN PATENTS 753,999 Great Britain Aug. 1, 1956

